In a wireless cellular communications system, multiuser multiple-input multiple-output (MU-MIMO) is a promising technique to significantly increase the cell capacity. In MU-MIMO, the signals intended to different users are simultaneously transmitted with orthogonal (or quasi-orthogonal) precoders. On top of that, the concept of a joint optimization of multiuser operation from both transmitter and receiver's perspective has the potential to further improve multiuser system capacity even if the transmission/precoding is non-orthogonal which could come from, for example but not limited to, the simultaneous transmission of a large number of non-orthogonal beams/layers with the possibility of more than one layer of data transmission in a beam. Such non-orthogonal transmission could allow multiple users to share the same resource elements without spatial separation, and allow improving the multiuser system capacity for networks with a small number of transmit antennas (i.e. 2 or 4, or even 1), where MU-MIMO based on spatial multiplexing is typically limited by wide beamwidth. An example of such joint Tx/Rx optimization associated with adaptive Tx power allocation and codeword level interference cancellation (CW-IC) receiver is recently a remarkable technical trend, including non-orthogonal multiple access (NOMA) and other schemes based on superposition coding.
In LTE, the rate-matching algorithm repeats or punctures the bits of a mother codeword to generate a requested number of bits according to the size of the time-frequency resource and a desired code rate that may be different from the mother code rate of the channel encoder. Besides, rate matching also needs to take into account the soft buffer size of a code block at the receiver if soft packet combining is to be used to enhance the decoding performance. When superposition coding is used, a transport block intended to one user may also need to be decoded at another user's receiver. Nevertheless, according to the specifications of LTE, the soft buffer size per code block depends on the UE category. Therefore, the soft buffer sizes of the two receivers may not be the same. To enable the superposition coding in LTE, some issues related to rate matching need to be investigated. Specifically, two issues related to the soft buffer size setting at the transmitter and soft channel bits storage at the receiver in the superposition coding are discussed and resolved.
Furthermore, the soft buffers in the receiver are reserved for the desired signal as well as the interfering signals to be handled by CW-IC. As a result, the UE and the network should have the same understanding on the partition of the soft buffer for the two types of signals. Otherwise, the soft channel bits of some transport blocks (TBs) may not be accumulated, and soft packet combining of multiple (re)transmissions does not work efficiently. Up to LTE Rel-12, the soft buffer is reserved only for desired TBs, and the partition schemes for TBs is related to the number of DL HARQ processes. Thus, a method of soft buffer partition and restricting a total number of active DL HARQ processes for both desired and interfering TBs in superposition coding is proposed.